classic template - blue · 8 dust classification based on k st measured k st classification 300...
TRANSCRIPT
1
Objectives
• OSHA - Enforcement
• Define the terminology
• Recognize a Dust Explosion
hazard
• Learn hazard management
methods
Enforcement
• OSHA Established an NEP for
Combustible Dust
• Started the Standard Process
– Holding Stakeholder Meetings
– Gathering Information
– Proposed Standard – Not sure when this will
be done (No specifics at this time)
2
Types of Industries Involved in Dust Incidents
Types of Dust Involved in incidents
Definitions and Terminology
• Combustible Dust
• Deflagration
• Minimum Ignition Energy
(MIE)
• Kst
3
Combustible Particulate Solids
Chunks
Flakes
Fibers
Chips
Fines
Dusts
“Combustible Dusts”
are a sub-category of
“Combustible Particulate Solids”
Combustible Dust
• “A combustible particulate solid that presents a
fire or deflagration hazard when suspended in
air or some other oxidizing medium over a range
of concentrations, regardless of particle size or
shape.” [NFPA 654-2006]
• Dusts are just a small fraction of combustible
particulate solid.
• This definition does NOT encompass all of the
hazardous particulates!
Combustible Dust
420
1.3 mm
A particle that will pass a #40 Sieve
A fiber particle
12
4
Particle Size of Common Materials
Deflagration
• Propagation of a combustion zone at a velocity that is less than the speed of sound in the unreacted medium.
• This action releases large quantities of heat very rapidly.
• This in turn produces large pressures to develop that can lead to structural failure and cause employee injuries.
Deflagration
Dust cloud
5
Deflagration
Deflagration
Deflagration
6
Deflagration
Minimum Ignition Energy
• MIE is the minimum energy, discharged
into the dust cloud by the test apparatus
and sufficient to cause flame propagation.
The MIE is measured in units of joules (J)
or millijoules (mJ).
Minimum Ignition Energy
Increasing particle
size increases the MIE,
decreasing the particle
size decreases the
energy necessary to
initiate deflagration.
7
Minimum Ignition Energy
MIE varies with
concentration. MIE
is reported at the
concentration that
deflagrates with the
lowest MIE.
Kst
• Volume-Normalized Rate of Pressure Increase.
• KSt is also referred to as the “Deflagration Index” or
“Dust Constant”. It is calculated as the rate of
pressure increase multiplied by the cube root of the
test vessel volume. KSt = (dP/dtmax)(V)1/3
• KSt is proportional to the speed of the flame front
through the dust cloud.
• KSt is measured in units of bar-meter per second (bar-
m/sec). KSt is just a way of classifying dust to estimate
the anticipated behavior of the dust during
deflagration.
KSt
• Dusts with a KSt less than or equal to 200 bar-m/sec are classified as “ST-1” dusts.
• Dusts with a KSt between 200 and 300 bar-m sec are classified as “ST-2” dusts.
• Dusts with a KSt greater than 300 bar-m/sec are classified as “ST-3” dusts.
• These classifications have NOTHING to do with hazardous area classifications under the NEC.
8
Dust Classification Based on KSt
Measured KSt Classification
<=200 St-1 *
200 – 300 St-2
>300 St-3 **
• * THIS CLASSIFICATION KILLS THE MOST EMPLOYEES
• ** THIS CLASSIFICATION IS USUALLY MADE UP OF METALS – Al, Mg,
Be, etc.
Combustible dusts are classified by the
numerical value of KSt.
Recognition
•Review the materials in the facility that are utilized and/or manufactured. If any
the material is reduced to fines, dust, chips, chunks, flakes or fibers then research
the material further because it may be a combustible dust.
•Keep in mind, any material containing “carbon” and/or un-oxided metal will most
likely explode.
•Check your material safety data sheet (MSDSs) concerning any unusual hazards
such as explosibility.
•Review the online German database
http://www.dguv.de/bgia/en/gestis/expl/index.jsp. The database is operated by
BGIA - Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung. It
contains combustion and explosion characteristics of more 4000 dusts samples
from virtually all sectors of industry.
•If you are unable to locate any information about your material, then try using
Google.com to look for such hazards.
Recognition (cont.)
• Take a sample of the dust in your plant from the highest points when possible. Have the material tested for its KSt. This will at least give you an idea whether the material will explode.
• Many locations need to be considered in an assessment. One obvious place for a dust explosion to initiate is where dust is concentrated. Equipment such as dust collectors may contain a combustible mixture whenever the equipment is operating.
• Other locations to consider are those where dust can settle, both in occupied areas and in hidden, concealed spaces. A thorough analysis would include all possible scenarios in which dust can be disbursed, both in the normal process and potential failure modes.
• A hazard may exist without visible dust in the air. Meaning an initial explosion may occur in dust collector and create secondary explosion in areas with poor housekeeping. A sure sign of close encounters may be a plant history of fires.
• While evaluating the dust in your facility, keep in mind the potential ignition sources such as flames, sparks from welding/cutting operations, friction or impact sparks, electric sparks, electrostatic discharges and hot surfaces.
9
Combustible Dust Explosion
Pentagon
Fire Triangle - Dust Explosion Pentagon
FIRE TRIANGLE:
DUST EXPLOSION
PENTAGON:
Combustible Dust Oxygen in Air
Ignition Source
Dispersion Confinement
Explosion
1. Combustible Dust 2. Oxygen in Air
5. Ignition Source
3. Dispersion 4. Confinement
Explosion
1. Combustible
Dust
2. Oxygen in Air
3. Dispersion
4. Confinement
5. Ignition Source
IMPORTANT
NO DUST EXPLOSION OCCURS
if one or more elements
are missing
Combustible Dust Explosion Pentagon:
Five Elements – ALL Necessary
10
Element 1: Combustible
Dust
Agricultural Products such as:
• Corn Starch, Dry Milk, Sugar,
Wood Flour, Powered Milk
Agricultural Dusts such as:
• Cocoa Powder, Hops (malted),
Rice Flour, Wheat grain dust
Carbonaceous Dusts such as:
• Petroleum Coke, Pine Soot,
Bituminous Coal, Wood
Charcoal.
Chemical Dusts such as:
• Lactose, Sulfur, Calcium
Acetate, Methyl-Cellulose
Plastic Dusts such as:
• Phenolic Resin,
(poly)Propylene, (poly)Vinyl
Chloride, Melamine Resin
Metal Dusts such as:
• Aluminum, Magnesium, Zinc,
Bronze
Explosion
Combustible Dust Particles
may
break
into
or
Combustible Dust Particles
• Size – Pellets > 2mm diameter
– Granules 0.42mm – 2mm
– Dust Particles <0.42mm
11
Combustible Dust Particles
• Hazard increases as particle
size decreases
– Larger surface area for combustion
– Fine particles may have a larger
role in dust cloud ignition and
explosion and propagation
Combustible Dust Particle Size
Distribution
Decreasing Particle Size Increases dP/dtmax
EckhoffEckhoff
Aluminum Dust
dP
/dtm
ax [b
ar/
sec]
Explosion
Element 2: Oxygen in Air
• The Oxygen content in air is all that is
necessary to support an explosion.
12
Explosion
• Dust needs to be dispersed in the air
Element 3: Dispersion
Combustible Particulate
Largest particles
Smallest particles
•Generally the finest (most hazardous) fraction of the accumulated fugitive
dust settles highest in the compartment of building.
•Every large compartment is a particulate separator, separating particles
by mass.
Concentration
• Fact: When particles are suspended, a
concentration gradient will develop where
concentration varies continuously from high
concentration to low concentration,
Concentration
Gradient
Ignitable
Non-ignitable
Dust
Cloud
13
What
bulb?
25 watts
6 feet
Glass
40 g/m3 concentration of
comb. dust suspended in air
Glass
Dust Combustibility
• 25 watt light bulb probably can not be seen
through six feet of a mixture of combustible dust
in air > Minimum Explosible Concentration
Dust Combustibility
0
2.5 ug/m315 mg/m
3
50 g/m3
3000 g/m3
Explosible
Range
Health
Hazard
Range
Concentration 3000X
Explosion
Element 4: Confinement
• Confinement can be provided by buildings,
process equipment, ducting, piping, and
dust collection equipment.
14
Explosion
Element 5: Ignition Source
Can be Mechanical
• Match/lighter
• Spark
Can be Electrical
• Static –• Human - 20-30 mj
• Grain – filling – 10-25 mj
• Lightning
• Generated
Explosion
All 5 Elements = EXPLOSION
The “Typical” Explosion
Event
• Dust explosions occur as a series of
deflagrations leading to a series of
explosions in stages.
– While a single explosion is possible it is the
exception rather than the rule.
– Most injuries are the result of the “secondary”
deflagrations rather than the initial event.
15
A Dust Explosion Event
Primary Deflagration inside process equipment
0 25 50 75 100 125 150 175 200 225 250 300 325
Time, msec.
(Timing of actual events may vary)
A Dust Explosion Event
Shock wave caused
by primary deflagration
Time, msec.
0 25 50 75 100 125 150 175 200 225 250 300 325
A Dust Explosion Event
Shock waves reflected by surfaces within the building
cause accumulated dust to go into suspension
Time, msec.
0 25 50 75 100 125 150 175 200 225 250 300 325
16
A Dust Explosion Event
Time, msec.
0 25 50 75 100 125 150 175 200 225 250 300 325
Dust clouds thrown in the air by the shock waves
A Dust Explosion Event
Time, msec.
0 25 50 75 100 125 150 175 200 225 250 300 325
Primary deflagration breaks out of the equipment
enclosure – creating a source of ignition
A Dust Explosion Event
Time, msec.
0 25 50 75 100 125 150 175 200 225 250 300 325
Secondary deflagration ignited
17
A Dust Explosion Event
Time, msec.
0 25 50 75 100 125 150 175 200 225 250 300 325
Secondary deflagration is propagated
through the dust clouds
Time, msec.
0 25 50 75 100 125 150 175 200 225 250 300 325
Secondary
deflagration bursts
from the building
A Dust Explosion Event
Time, msec.
0 25 50 75 100 125 150 175 200 225 250 300 325
Collapsed building with remaining fires
18
Equipment
& Operations
Blenders/Mixers
• Heat Generation due to
– Rubbing of Solids
– Rubbing of internal parts
• Electrostatic Charging of the Solids
• Dust Formation inside of the equipment Source:
http://www.fedequip.com/abstract.asp?ItemNumber=1
7478&txtSearchType=0&txtPageNo=1&txtSearchCriter
ia=ribbon_mixer
Dryers
• Direct-Heat Dryers– Convective Drying
System
– Heat provided by heated air or gas
– Moisture is carried by drying medium
• Indirect–Heat Dryers– Heat transfer by
Conduction
– Steam for Jacketed Dryers
Source:www.barr-rosin.com/products/rotary-dryer.asp
19
Dust Collectors
• Presence of easily ignitable fine dust atmosphere and high turbulence
• Experienced many fires over the years due to broken bags.
• Ignition source is electrostatic spark discharges
• Another ignition source is entrance of hot, glowing particles into the baghouse from upstream equipment
Fabric Filters (Baghouses)
Dust Collector
• If burning material is introduced into the dust collector, a deflagration can result from the operation of the bag or filter element cleaning cycle.
• Dust collector system located inside a building
Note: A dust collector, by its very operation, maintains a cloud of finely divided particles suspended in air. If a source of ignition initiates the combustion of the dust cloud, the collector casing could cause a violent rupture. When dust particles are known to be combustible, precautions for an explosion must be taken and suitable protection provided to reduce the risk of personal injury.
Dust explosion in
equipment
With dispersal
and ignition of 2 kg dust
by the flame jet
Dust
Collector
Dust explosion in
equipment
20
Pneumatic conveying system
• Downstream equipment have high rate of risk for fires and explosion
– Static electricity is generated from particle to particle contact or from particle to duct wall contact.
– Heated particles which are created during grinding or drying may be carried into the pneumatic conveying system and fanned to a glow by high gas velocity.
– Tramp metal in the pneumatic system may also cause frictional heating.
– Charged powder may leak from joints to the atmosphere and electrostatic sparking can occur resulting in an explosion.
Figure source:www.flexicon.com/us/products/PneumaticConveyingSystems/index.asp?gclid=COa2kKWK
4o8CFQGzGgodikc9Dg
Size Reduction System
• Size reduction equipment is regarded as a possible ignition source because of friction and hot surfaces arising from grinding
• Entrance of metal into the equipment
• Too slow feed rate can increase the possibility of fire/explosion hazard
Bucket Elevators
• Belts Slipping
• Belts and lagging are not fire and oil resistant
• Belts and bearings are not outside the casing
• Lack of preventive maintenance
Head
pulley
Lagging
21
Silos and Hoppers
• No inter-silo Venting
• Silos and hoppers shall be located outside the buildings
with some exceptions
• Air cannons not to be used to break bridges in silos
• Detection of smoldering fires in silos and hoppers can be
achieved with methane and carbon monoxide detectors
• Pressure containment, inerting, and suppression
systems to protect against explosions
• Venting is the most widely used protection against
explosions
Explosion
Hazard
Management
Methods
• There are three ways of handling combustible dust
hazards:
– Prevention
• Explosion suppression
• Dilution with non-combustible dust
• Oxidant concentration reduction
– Mitigation• Controlling the explosion energy
• Venting through a listed dust retention and flame arresting
device
– Combination of Prevention and Mitigation
22
Processes
• Combustible particulate solids should not
pass through fans.
• All conveyance ducts shall be steel or
other conductive material.
• All bends smooth.
• Changes in diameter with 10o(max) taper
transitions.
Processes
This Not this
This Not this
Explosion Venting
• The vent opening must be sized to allow the expanding gases to be vented at a rapid rate so that the internal pressures developed by the explosion do not compromise the structural integrity of the protected equipment
• The volume of the equipment to be protected
• The maximum pressure during venting (Pred)
• The KSt of the dust (or fundamental burning velocity of a gas)
• The burst pressure of the explosion
23
Deflagration Relief Venting (Explosion Venting)
Dust Collector
Filter Media
Dusty Air
Into Collector
Return Air
Vent
Deflagration Relief Venting (Explosion Venting)
Dust Collector
Filter Media
Dusty Air
Into Collector
Return Air
Vent
Deflagration Relief Venting (Explosion Venting)
Dust Collector
Filter Media
Dusty Air
Into Collector
Return Air
Vent
24
Deflagration Relief Venting (Explosion Venting)
Dust Collector
Filter Media
Dusty Air
Into Collector
Return Air
Vent
Deflagration Relief Venting (Explosion Venting)
Dust Collector
Filter Media
Vent
Vent Duct
Wall
Deflagration Suppression
• Reaction times are generally in the 30 to
40 millisecond time domain.
• Pressure sensors are generally calibrated
to 0.5 psi above ambient.
• Extreme care should be used when
Deflagration Suppression is used on
vessels also equipped with Deflagration
Relief Vents.
25
Deflagration Suppression
• Must be designed, installed and
maintained per NFPA 69.
• Relies on high speed pressure sensors
and high rate discharge extinguishing units
to detect initial shock wave from the
deflagration and discharge suppressing
agent before the pressure has gotten large
enough to cause damage.
Deflagration Suppression
Specialized Agents
• Gaseous agents
• Dry chemicals
• Water spray or mist
• Others
Pressure Sensor / Automatic Dust Explosion
Suppression System
26
Detectors
Detection & Supression System
Deflagration Suppression
Dusty Air
Into Collector
Return Air
Pressure
Sensors
Control
Unit
HRD
Extinguishing
Unit
27
Deflagration Suppression
Dusty Air
Into Collector
Return Air
Pressure
Sensors
Control
Unit
Deflagration Suppression
Dusty Air
Into Collector
Return Air
Pressure
Sensors
Control
Unit
HRD
Extinguishing
Unit
Deflagration Suppression
Dusty Air
Into Collector
Return Air
Pressure
Sensors
Control
Unit
HRD
Extinguishing
Unit
28
Spark Detection and Extinguishment
• Spark detectors are sensitive infrared
sensors that detect burning material on a
conveyor or in a pneumatic conveyance
duct.
• Spark detection is usually used in
conjunction with automatic water-spray
extinguishment.
Spark Detection and Extinguishment
• When the spark enters the field of view of
the detector it sends a signal to the control
unit which energizes the valve.
• The valve allows a spray of water to
commence in the duct.
• The spark collides with the water spray
and is quenched.
Dust Collector
Filter MediaReturn Air
Spark Detection and Extinguishment
Fan
Control
Unit
Pump
Solenoid
Valve
Spark
Detectors
Vent
29
Dust Collector
Filter MediaReturn Air
Spark Detection and Extinguishment
Fan
Control
Unit
Pump
Solenoid
Valve
Spark
Detectors
Vent
Dust Collector
Filter MediaReturn Air
Spark Detection and Extinguishment
Fan
Control
Unit
Pump
Solenoid
Valve
Spark
Detectors
Vent
Dust Collector
Filter MediaReturn Air
Spark Detection and Extinguishment
Fan
Control
Unit
Pump
Solenoid
Valve
Spark
Detectors
Vent
30
Dust Collector
Filter MediaReturn Air
Spark Detection and Extinguishment
Fan
Control
Unit
Pump
Solenoid
Valve
Spark
Detectors
Vent
Deflagration Relief Venting – Flame Arrestor
• Deflagration through a listed dust retention and flame-arresting device
Cut-away of deflagration vent
• Burned and unburned dust is
retained by filter and flame arrestor
• Combustion gases are cooled
• No flame emerges from the system
• Nearby blast effects are reduced
• May be vented indoors
• Change burst disc and clean filter
after each event (Europe)
No Isolation Device
31
Isolation Devices for Ductwork
Ventex ValvePassive device for interrupting dust explosion
Bursting Disc /or
other Vent CoverDevice for interrupting dust explosion
Back Flash Dampers for DuctworkIsolation Devices
Chemical & Mechanical Isolation
Devices
32
Isolation Devices
• Airflow control valve, NFPA 654 (7.8)
• Diverter valves seal mechanically and close all
other directions from air or material leakage
Diverter
valve
Normal flow
of material
Positive shut
off flap
Flame
front
Hinged
device
Isolation Devices• Segregation of the hazards (isolate with a diverter)
Flame
front
diverter
Roof Line
Air
Isolation Devices
• Segregation of the hazard (isolate with a diverter)
33
Isolation Devices
• Segregation of the hazard (isolate with a barrier)
Automatic Fast Acting Isolation Valve Is Shown
Isolation Devices
• Segregation barrier – Chemical Suppressor
Isolation Devices
• Segregation of the hazard (chemical isolation)
Flame front is
halted by barrier
released from
chemical
suppressor
34
Without Return Air Abort
• As the flame and hot gases fill the space
beneath the ceiling/roof deck sprinkler
heads begin fusing.
• Often more heads fuse than the riser is
designed to support.
• The excessive demand deprives entire
facility of required delivered density and
fires are not controlled.
Dust Collection Without
Return Air Abort
Dust
Collector
Return Air Duct
Radiant flux
Dust Collection Return Air Abort
• The diversion of return air to
building exterior is usually
implemented with spark
detection and a fast-acting
abort gate.
35
Dust Collection Return Air Abort
Dust
Collector
Return Air Duct
Abort Gate
Return Air Abort
• Upon actuation of the return air spark
detection the automatic bag cleaning is
shut-down.
• DO NOT shut down the air movement as
this can cause a deflagration
• If fire expands the sprinklers in the D/C will
fuse and dissipate the heat generated by
the fire.
Return Air Abort
• In lieu of using an abort gate,
fast acting valves can be used
to prevent fire and smoke
from returning to the occupied
space.
36
Dust Collector
Filter Media
Dusty Air
Into Collector
Return Air
Spark Detectors
Control
Unit
NFPA 664-2007 Section A 8.2.2.6 Minimum Compliance Design
Vent
Valve or
Flame-front
Diverter
Chokes for Screw Conveyors
Isolation Devices
Rotary Valve
Isolation Device
Rotary
Air Lock
37
Explosion Management – Pressure
Containment
Vessel must contain the
calculated maximum explosion
pressure equal to
2/3 yield
strength
Allows vessel
re-use
2/3 catastrophic
failure pressure
Contains the
event, but the
vessel must be
scrapped
Wet Scrubber/Collectors
• Air is drawn into the
collector and is forced
to churn through a
torturous path,
through a partially
submerged
baffle. Dust is
separated by making
contact with water in
this section.
Process Equipment - Bucket Elevators
• Deflagration venting
• Inside bucket elevators must be provided with deflagration venting ducted to the outside or otherwise properly protected against explosion
• Elevators must be dust tight and noncombustible
• Inlet and discharge hoppers are to be accessible for cleaning and inspection deflagration
venting test at
mock-up lab
38
Bucket Elevators
• Power cutoff
• Is a device used to cut off
power when the drive motor
drops to <80% of normal
• Feed is to be stopped or
deflected when the power to
the motor is stopped
Bucket Elevators
• Belts
• Non-slip material and (lagging) is to be provided on the head pulley to minimize slippage
• Belts and lagging are to be fire and oil resistant
Head
pulley
Lagging
Bucket Elevators
• Belts
• Bearings are to be outside the casing
• Provide openings at the head and tail pulley for
– Cleanout
– Inspection
– Alignment
Dirty
Really dirty and
“partially
inside”
construction
39
Bucket Elevators
• Drive components are
to be engineered to:
– Handle full rated
capacity
– Start the unchoked
elevator under full load
Bucket Elevators
• Monitors are to be provided
• Monitor the head and tail pulleys for:– High bearing temperature
– Vibration
– Head pulley and belt alignment
• An alarm requiring corrective action to sound at the operator’s station
• Exception: Conveyors <500 ft/minor <3750 ft3/hour
Head
Tail
Bucket Elevators
• Emergency Controls
• Bins where material directly discharges from the elevators must be provided with:
– Automatic high-level shutdowns
– Visual indicators
– Audible alarms sounding at the operator control station
40
Explosion Management
• Oxidant Concentration
Reduction
• Outside the combustible range
through oxidant concentration
reduction, fuel enrichment or
both
• Detailed engineering
requirements
• Life safety hazard
• Metals problematic
Normal
Limiting
Oxygen
Concentration
for combustion
of many dusts
Explosion Prevention Techniques
• Elimination of Ignition Sources Involves:
– Control of Heat Sources -NFPA 654
– Control of Friction -NFPA 654
• Mechanical friction
• Friction sparks
– Control of Electrical Sparks -NFPA 497, NEC
– Control of Static Electricity -NFPA 77
Ignition Control
• Separate heating systems
from dusts
• Proper use of cartridge
activated tools
• Adequately maintain
equipment.
Hot
work
permit_________
_________
_________
_________
Kaboom!
41
Ignition Control
• Control mechanical sparks and friction;
• Use separator devices to remove foreign materials capable of igniting combustibles from process materials;
• Separate heated surfaces from dusts;
Magnetic
Core
Non - Magnetic
rotating drum
Ignition Control
• Use appropriate electrical
equipment and wiring
methods
• Control static electricity,
including bonding of
equipment to ground
• Control smoking, open
flames, and sparks
No!
42
Ignition Control
• Other ignition sources
– OSHA 29 CFR 1910.178 (c)
regulates powered industrial
trucks in dust areas
– Coal handling operations
must comply with OSHA 29
CFR 1910.269
Class I & II
Group D & G
Explosion Prevention Techniques - Control of Heat
Sources
• If the material is subjected to heat as part of the normal process (e.g. during drying), the temperature should be maintained below the self heating temperature (for solids)
• Preventing the overloading of processing plant (grinders, conveyors, etc.). Internal buildup will BOTH reduce heat loss from material AND increase operating temperature above “normal”. Consider the installation of overload protection devices on drive motors
• Isolation or shielding of hot surfaces
• Prevention/removal of dust accumulations on hot surfaces
• Use of approved electrical equipment (correct temperature rating)
Explosion Prevention Techniques
Control of Friction
Mechanical equipment and component
could be a source of ignition due to:
– Mechanical impacts producing:
• Small flying fragments of hot/burning material
• A pair of hot spots where impacting bodies touch
– Mechanical friction caused by objects rubbing against each
otherand producing hot surfaces
– Thermitereaction caused by impacts involving aluminum
and rust
43
Explosion Prevention Techniques
Control of Friction
• Prevent overheating due to misalignment, loose objects, belt-slip/rubbing etc. by regular inspection and maintenance of plant
• Prevent foreign material from entering the system when such foreign material presents an ignition hazard. Consider use of screens, electromagnets, pneumatic separators, etc.
• Floor sweepings should not be returned to any machine
• Impact sparks can occur when for example operators use, drop, orotherwise strike metal equipment with metal tools or objects. Minimize the likelihood of impact sparks through:
– Proper tool selection
– Techniques to prevent dropping tools e.g. wrist straps
– Operator training
• Hot work operations should be controlled by a hot work permit system in accordance with NFPA 51B, Standard for Fire Prevention During Welding, Cutting and Other Hot Work
– Formation of dust clouds should be prevented, and dust deposits should be removed
– A gas/vapor detector may be used to ensure flammable vapors/gases are not present
Explosion Prevention Techniques –
Control of Static Electricity
Contact (Frictional) Charging
Electrostatic charges are usually generated when any
two materials make and then break contact
The build up of the charge on electrically isolated
conductors and/or on insulating materials, can give
rise to electrostatic discharges
Depending on the energy of the discharge, a
flammable atmosphere can be ignited.
44
Electrical Area Classification of Dust
Locations
• The following factors determine the extent of Class II locations:– Combustible material involved
– Bulk density of the material
– Particle sizes of the material
– Density of the particles
– Process or storage pressure
– Size of the leak opening
– Quantity of the release
– Dust collection system
– Housekeeping
– Presence of any flammable or combustible
gas
Housekeeping
• Facilities shall be maintained dust free.
• No blow-down unless ALL electrical power
and processes have been shut-down.
• No welding, cutting or grinding unless
under a hot-work permit per NFPA 51B.
• Comfort heating equipment shall obtain
combustion air from a clean outside
source
Housekeeping – Rule of Thumb
• If you can write
your name in
the dust there
is probably
sufficient dust
present to
blow the
building away.
45
Housekeeping
•The majority of the property damage and
personnel injury is due to the fugitive dust
accumulations within the building or process
compartment.
•Control, limitation of elimination of
accumulated fugitive dust is CRITICAL
and the single most important criterion for a
safe workplace.
Housekeeping
• Include:– Overhead beams, joists, ducts,
– Tops of equipment, and other
surfaces
– Vertical surfaces if they are dusty
– Structural members
– Conduit and pipe racks
– Cable trays
– Floors
– Above ceiling tile
– Equipment (leaks around dust
collectors and ductwork.)
?
Housekeeping
• Some recommendations:– Minimize the escape of dust from
process equipment or ventilation
systems to minimize the time
needed for housekeeping
– Use dust collection systems
– Utilize surfaces that minimize
dust accumulation and facilitate
cleaning Spots are
not
raindrops
46
Housekeeping
• Develop and implement a hazardous dust plan– Inspection
– Testing
– Housekeeping
– Control program
• Program should be written with established frequency and methods
Housekeeping
• Use cleaning methods
that do not generate dust
clouds
• Only use vacuum
cleaners approved for
dust collection
HAZ LOC
Dust Layer Thickness Guidelines
1/8” in grain standard
Rule of thumb in NFPA 654
1/32” over 5% of area
Bar joist surface area ~ 5%
Max 20,000 SF
Idealized
47
Training
• They need to know
– The safe work practices applicable to their job tasks
– The overall plant programs for dust control and ignition source control
• Training must be
– Before they start work
– Periodically to refresh their knowledge
– When reassigned
– When hazards or processes change
Training
• Training needs were identified for:
– Employees
– Supervisors
– Managers
Electrical
48
Hazardous (classified) Locations
1910-307
Subpart S
Electrical – Design
Safety Standards
for Electrical Systems
29 CFR 1910.307(c) Electrical
Installations
• Equipment, wiring methods, and installations of equipment in hazardous (classified) locations shall be intrinsically safe, approved for the hazardous (classified) location, or safe for the hazardous (classified) location !
Electrical Equipment for
Hazardous Occupancies
• All electrical equipment must be “listed” for
use in the occupancy based upon the
Class, Division and Group classification.
• When all electrical equipment in the
occupancy is listed for use in that
occupancy the electrical system is not
deemed to be a likely igniter.
49
Class II Hazardous Occupancies
• Occupancies containing combustible dusts.
• Division 1: – Dusts suspended in the air under normal operating conditions.
– Production system upset, equipment failure or maintenance can produce BOTH dust suspension and ignition source.
– Where combustible dusts are electrically conductive.
• Division 2:– Compartments where ignitable dust suspensions do not normally
exist outside process equipment.
– Where dust accumulations are not normally sufficient to cause electrical equipment to overheat.
Class II Locations
Class II locations are those that are hazardous because of the presence of combustible dust. The following are Class II locations where the combustible dust atmospheres are present:
Group E. Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, and other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment.
Group F. Atmospheres containing combustible carbonaceous dusts that have more than 8 percent total entrapped volatiles (see ASTM D 3175, Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke, for coal and coke dusts) or that have been sensitized by other materials so that they present an explosion hazard. Coal, carbon black, charcoal, and coke dusts are examples of carbonaceous dusts.
Group G. Atmospheres containing other combustible dusts, including flour, grain, wood flour, plastic and chemicals.
Group E Dust
• Could cause a short in the electrical equipment
• Electricity may find the path of least resistance through a dust layer, heating up the dust particles in it path and thus providing a source of ignition. The resulting electric arc could ignite a dust layer or dust cloud.
*NFPA 499 . . . Hazardous (Classified)
Locations (2008) Sec 4.4
50
Class II, Division Locations
• Equipment must be marked to identify suitability
for use with Group E, F and/or G.
• Where suitable for ambient temperatures
exceeding 40º C (104º F) marked with both
– The maximum ambient temperature and
– The operating temperature or temperature range at
that ambient temperature.
Nationally Recognized Testing
Laboratories (NRTLs)
• NRTLs are third party organizations recognized by OSHA
• Provide product safety testing and certification services to manufacturers on a wide range of products
• Testing and certifications are based on product safety standards developed by U.S. based standards developing organizations
• Often issued by American National Standards Institute (ANSI)
51
NFPA 654 Guide to Area Electrical
Classification – A.6.6.2Negligible
dust -color
discernable
Not much dust
being released
N/A Unclassified –
general purpose
Negligible
dust to <
1/32 inch
(paper clip)
Infrequent –
Episodic release,
not > 2-3 times
yearly
Clean up
during same
shift
Unclassified –
general purpose
Negligible
dust to <
1/32 inch
Continuous to
frequent –
Continuous with
< 1/32 inch per
24 hours or
Episodic release,
> 3 times yearly
Clean as
needed –
maintain
average <
1/64 inch
(puffy cloud
with each
step)
Unclassified – dust
tight NEMA 12
enclosures and
/sealed non- heat
producing
enclosures (For
existing plants – New
to be Class II, Div. 2)
NFPA 654 Guide to Area Electrical
Classification – A.6.6.2
1/32 to
1/8 inch
(Two
Quarters
stacked)
Infrequent Clean up during
same shift
Unclassified –
Dust tight NEMA 12
enclosures and /sealed
non- heat producing
enclosures
(For existing plants – New
to be Class II, Div. 2)
1/32 to
1/8 inch
Continuous
to frequent
Clean as needed –
maintain average <
1/16 inch
Class II, Division 2
>1/8 inch Infrequent Immediately shut
down and cleanClass II, Division 2
> 1/4 inch Continuous
to frequent
Clean frequently –
minimize
accumulation
Class II, Division 1
Electrical Classification and
Cleaning
• If Clean-up is constant and– Dust layer is not apparent;
– Surface color is discernible;
– e.g.: Storage area with bags,
drums, or closed hoppers;
– No dust around.
Classify as
unclassified
Electrical
Equipment
Metal Dust is always
Class II Div I